Warm Gulf waters spawned Hurricane Michael's intensity -scientists

Oct 10 (Reuters) - When Hurricane Michael barreled into the Florida Panhandle on Wednesday with 155-mph (249-kph) sustained winds, it defied forecasts made just two days beforehand, its wind speeds doubling since Monday and coming in just short of the highest category of intensity.

Michael was not the only tropical cyclone in recent years to undergo what scientists refer to as “rapid intensification,” defined as an acceleration of wind speeds of at least 35 mph (56 kph) in 24 hours or less. The phenomenon has become more serious as sea waters have warmed with climate change.

“It is most likely that the very warm water in the Gulf ... is likely contributing to the intensity and the intensification that we have seen,” said Jim Kossin, an atmospheric scientist with the U.S. National Oceanic and Atmospheric Administration.

Rapid intensification is dangerous because it gives businesses, industry and people on the ground less time to take appropriate precautions ahead of hurricanes like Michael that hit shore with far more furious winds than originally expected.

Several factors can contribute, but warmer waters increase the potential for a storm becoming stronger, climate scientists have said.

Climate change means that warmer ocean temperatures are being seen more often, said Jennie Evans, a professor of Meteorology at Pennsylvania State University.

“Since we’ve got warm temperatures in the Gulf now, a hurricane has a much better chance to reach its maximum possible intensity, which appears to be what Michael is doing,” she said.

In some cases, high-altitude winds can put a cap on a storm’s ability to draw power from moist sea air. But in Michael’s case, that brake was not in place.

Rapid intensification happens about 5 percent of the time and is difficult to predict. Last year, in the Atlantic basin, forecasters correctly forecast six of 39 such instances, according to Michael Brennan, branch chief of the hurricane specialist unit at the U.S. National Hurricane Center.

The NHC said last year it failed to adequately predict the rapid intensification of Hurricane Matthew in 2016 to a Category 5 storm with maximum sustained winds of 165 mph (270 kph). Matthew was directly responsible for 585 deaths, including more than 500 people in Haiti, the NHC said.

While the science of tracking a storm relies heavily on data about conditions on its periphery, predicting intensity relies on measuring what is happening in the middle of it.

Typically, that means flying a hurricane hunter aircraft inside the storm, recording wind speeds from a weather buoy as a storm passes overhead or relying on satellites that may fly over once every other day.

Collecting consistent data through the storm is difficult and limits the ability to predict intensity, hurricane experts have said.

In recent storms that have intensified quickly, the phenomenon has been more exaggerated than in the past, according to a recent research paper co-authored by Pacific Northwest National Laboratory researcher Karthik Balaguru.

“At the end of the day, deep warm water, what we call ocean heat content, that’s really the fuel supply for these storms,” said J. Marshall Shepherd, director of the atmospheric sciences program at the University of Georgia.

Reporting by Jon Herskovitz in Austin, Texas; Additional
reporting by Gabriella Borter and Gina Cherelus in New York;
Editing by Frank McGurty and Peter Cooney